Applicants claim priority under 35 U.S.C. xc2xa7119 of GERMAN Application No. 198 34 153.9 filed Jul. 29, 1998. Applicants also claim priority under 35 U.S.C. xc2xa7120 of PCT/EPT99/05233 filed on Jul. 22, 1999. The international application under PCT article 21 (2) was not published in English.
The invention relates to a method and a number of devices for carrying out said method for evaluating the signals of magnetoresistive sensors, in connection with which for the purpose of eliminating an interference signal that is proportional to the zero offset, the magnetization of the magnetoresistive resistor strips is periodically adjusted or only modulated by flip current pulses alternating in the positive and negative directions by way of an integrated flip line or by an external flip coil. The magnetoresistive sensor can be employed in this connection for high-resolution measurements of a magnetic field or of a magnetic field gradient, or of values based thereon, for example for potential-free current measurements.
Measuring devices for measuring magnetic fields that employ such methods with the objective of separating the offset voltage caused in magnetoresistive sensor bridges by unevenness of the resistances and their temperature dependency, from the sensor signal that is proportional to the magnetic field, are already known. For example, laid-open patent specification DE-OS 34 42 278 describes a magnetic field measuring device that contains a magnetic field sensor with four sensor elements consisting of magnetoresistive layer strips with Barber pole structures. Said sensor elements are wired as a Wheatstone bridge and are fed by a direct current source. Owing to tolerances in the manufacture of the magnetoresistive layer strips, not all four bridge resistances are completely the same, and without the presence of a magnetic field to be measured, the offset voltage ensuing from the dc voltage value appears on the output of the sensor bridge. All of the magnetoresistive layer strips of the sensors each have in each case at the same time the same magnetically preferred direction ensuing from the direction of their respective magnetization. A magnetizing coil is arranged within the proximity of the sensor elements, which is supplied by a current pulse generator with short, alternating positive and negative current pulses. Said current pulses generate in the magnetizing coil an alternating magnetic field that reverses the magnetism of the magnetoresistive layer strips at the cycle of the flip current pulses. When a magnetic field to be measured is applied, an ac voltage appears on the zero branch of the sensor bridge in addition to the aforementioned dc offset voltage. Said ac voltage can be indicated by a phase-sensitive rectifier that is controlled by the current pulse generator. On the output, said rectifier has an integrator for eliminating the offset voltage contained as the ac voltage component. The output voltage of said integrator is proportional to the magnetic field associated with the magnetic field sensor as long as it lies within the linearity region of the magnetic field sensor.
An important drawback of the described measuring method consists in that magnetic fields can still be measured only with a maximal bandwidth amounting to about one hundredth up to about one tenth of the frequency of the flip current pulses because the offset voltage of the sensor bridge present in the signal as the alternating component downstream of the phase-sensitive rectifier is otherwise present in an excessive proportionate amount. A second drawback of the measuring method is conditioned by the limited linearity range of the characteristic of the sensor bridge, as well as by the temperature dependency of the gradient of the characteristic as well as its dependency on the component of the magnetic field acting in the longitudinal direction of the magnetoresistive layer strips. This leads to the fact that the proportionality of the output voltage of the described measuring circuit to the measured component of the magnetic field is available only within a highly limited range of the field intensity and only at a constant temperature and constant magnetic field component in the longitudinal direction of the strips.
Said second drawback is already no longer present in connection with an improved measuring method according to U.S. Pat. No. 5,351,005. In the circuit specified in said patent specification, a current is generated by phase-sensitive rectification that is proportional to the magnetic field intensity to be measured. Said current is fed into a compensating coil in which the magnetoresistive sensor bridge is located, and generates there a magnetic field. The circuit controls said current to a value that generates in the coil a magnetic field that just about cancels the magnetic field applied from the outside. The magnetoresistive sensor bridge still acts here only as a zero instrument. The non-linearities of the characteristic and their dependency on the temperature and the corresponding magnetic field component no longer play any role here. The output signal of the circuit is obtained, for example from a voltage drop that is generated by the current through the compensating coil on a fixed resistor. Two xe2x80x9csample and holdxe2x80x9d amplifiers are alternately employed, controlled at the cycle of the flip current generator, for measuring the respective voltage drop for the two directions of magnetization in the magnetoresistive sensor. The difference between the two voltages of the xe2x80x9csample and holdxe2x80x9d amplifiers, which is formed by a low-pass amplifier, still contains only the component of the signal that is proportional to the magnetic field, whereas the component that is proportional to the offset of the sensor bridge drops out due to the formation of the difference. The bandwidth with which magnetic fields can be measured based on the circuit described herein is substantially determined by the bandwidth of the low-pass amplifier. In the most favorable case it is possible to reach a bandwidth of half of the frequency of the flip current pulses because for obtaining the correct value of the magnetic field it is necessary that both xe2x80x9csample and holdxe2x80x9d amplifiers each have picked up at least one measured value. Another circuit for eliminating the bridge offset resulting from the sensor signal is specified in the xe2x80x9cData Handbook SC 17xe2x80x9d [1997] of Philips Semiconductors, page 36. In the present case, the bridge signal is first supplied to a differential amplifier. The dc voltage component of the signal downstream of the differential amplifier, which contains the offset voltage of the sensor bridge and the differential amplifier, is fed back negative to the input of the differential amplifier via a low-pass filter and is thus controlled to zero. The ac voltage component of the signal that is proportional to the magnetic field to be measured is rectified via a controlled amplifier. The rectified signal is fed as current into a compensating coil and cancels there the magnetic field to be measured. A filter is connected downstream because the jump-like change of the input voltage of the controlled amplifier by the normal deviation leads to voltage peaks at its output. This means that in the present case, too, the magnetic field can be measured only with a bandwidth that is far below the flip frequency.
The known methods proposed heretofore for eliminating the zero offset of the magnetoresistive bridge circuits all relate to those bridges whose resistor strips are provided with Barber pole structures, and whose magnetization can be adjusted in the positive or negative longitudinal direction by means of a dc or pulse field. The direction of the magnetic fields used in this connection substantially correspond with the light direction of the magnetoresistive resistor strips. The direction of the field to be measured extends perpendicular thereto in the heavy direction of the strips. However, patent specification U.S. Pat. No. 4,596,950 describes a magnetoresistive bridge that consists of four thin-layer strips made of anisotropic magnetoresistive material which do not carry any Barber pole structure. The cresistance of such thin-layer strips is quadratically dependent upon the intensity of the magnetic field in the heavy direction. Therefore, without application of a magnetic field, no field sensitivity is present with low field intensities unless an auxiliary magnetic field is applied to the thin-layer strips in the heavy direction for adjusting a working point. According to patent specification U.S. Pat. No. 4,596,950, said auxiliary magnetic field is jointly generated by the operating current of the bridge because the thin-layer strips forming the bridge resistors are arranged either in pairs one on top of the other, or a thin-layer line generating a magnetic field is present via the magnetoresistive thin-layer strips, with the operating current flowing through said thin-layer line. It is proposed in said patent specification to feed the bridge voltage to an ac voltage amplifier and to supply the output of the latter to a low-pass whose output signal is transmitted to an amplifier, and whose output current generates in a coil a magnetic field that compensates the field to be measured. The polarity of the operating current of the bridge has to be periodically changed in time for that purpose. The voltage drop caused on a known resistor by the compensating current is used as the output signal. Due to the simultaneous reversal of the direction of the operating current and the working point of the magnetoresistive tin-layer strips adjusted in that way, the output signal of not depending on the offset of the bridge.
The measuring arrangement proposed by said patent has the following drawbacks: The measurement is possible only up to an upper frequency limit that is below the limit frequency of the low-pass. Said limit frequency, however, may, at the most, come to about one tenth of the frequency at which the polarity of the operating current is changed. Furthermore, for adjusting the working point of the magnetoresistive thin-layer strips, permanently applied fields in the range of about 1 kA/m are required. Fields of that size can be realized in the described arrangements only with currents of a few 10 mA. Since the converted dissipation of a bridge has to be limited to about 0.1 watt at the most, only low bridge resistances can be used. This also limits the bridge voltages, specifically to a few tenths of one volt. The measuring sensitivity is highly restricted in that way, namely to about one hundredth of the value of a magnetoresistive bridge without correction of the offset voltage error. However, since the ratio of sensitivity to offset error needs to be as large as possible, only a minor improvement can be expected in this case.
As compared to the measuring methods specified above, which reverse the direction of the magnetization of Barber pole-structured magnetoresistive thin-layer strips with magnetic field pulses with alternating signs in the light direction, it has to be noted that the magnetic field pulse have to be applied only for a duration of less than one microsecond. This means that no restriction of sensitivity-conditioning parameters by electrical dissipation is existing in the present case.
Now, the problem of the invention is to propose a method for evaluating signals of magnetoresistive sensor elements with suppression of the bridge offset, whereby said sensor elements may form an array of sensor elements such as, for example a sensor bridge. With the method as defined by the invention, the direction of magnetization in the magnetoresistive sensor elements forming the resistors is periodically reversed by applying an alternating field, whereby the bandwidth of the evaluation method is not limited by the flip frequency employed for reversing the magnetism. Said problem is solved by the method that is described in the independent claim, and the method is further developed in specific ways as defined in the dependent claims.
The repeated reversing or modulating of the direction of magnetization in the magnetoresistive sensor elements addressed herein, which may be provided with Barber pole structures, by a magnetic pulse field with alternating directions, or with changing intensity; and by reversing the operating voltage of the sensor elements at the cycle of the magnetic pulse field, a signal is formed at the output of said sensor elements that is proportional to the magnetic field or magnetic field gradient acting on the sensor elements. Said signal is superposed by an alternating component with the offset signal as the amplitude. Said output signal is supplied to a differential amplifier, whereby in a simple exemplified embodiment of the invention, for example in an array of magnetoresistive sensor elements designed in the form of a Wheatstone bridge, the output voltage of the circuit is already available on the zero branch of the bridge as the measuring signal. For said output signal to no longer contain the alternating component conditioned by the offset, said alternating component is fed to the respective input of the differential amplified in a rectified and inphase form, so that it can be minimized by the differential amplifier, or theoretically even controlled to zero. For said purpose, the signal is transmitted from the output of the differential amplifier to a low-pass via a modulator or reversing switch controlled at the cycle of the magnetic pulse field, and then transmitted further via an amplifier to another modulator or reversing switch that is controlled at the cycle of the magnetic pulse field, said latter modulator or reversing switch establishing the connection to the respective input of the differential amplifier.
The low-pass in the feedback line may have a low limit frequency because offset changes in the sensor elements take place only at a relatively slow rate, for example due to changes in temperature, and are then nonetheless eliminated from the output signal by the control circuit. No provision is made for frequency-limiting stages in the actual signal path. This means that magnetic field changes to be measured are transmitted with high bandwidth as a change of the output voltage. Said bandwidth is not limited by the cycle frequency of the magnetic pulse field that serves for modulating or reversing the direction of magnetization of the magnetoresistive sensor elements, and is substantially greater than said frequency. The maximally transmittable frequency is limited by the fact that the magnetic field pulses for the above-described magnetization of the magnetoresistive layer strips must not fall short of a defined duration, and that during the change in magnetization, no change takes place in the resistance of the magnetoresistive layer strips that is proportional to the acting magnetic field.
According to a special, advantageous implementation of the method as defined by the invention, a proportional current is formed based on the output voltage of the differential amplifier and fed into a coil located within the proximity of the sensor bridge. At the site of the sensor bridge, the magnetic field of said current is directed against the magnetic field to be measured. The current is controlled by the circuit to a value that leads to cancellation of the magnetic field at the site of the sensor bridge. The offset voltage component at the output of the differential amplifier is controlled to zero in this process as described above, by rectification and inphase feedback to the input of the amplifier. The output signal of the circuit is, in the present case, the voltage drop that the current causes via a known resistor. Said drop in voltage is proportional to said output signal irrespectively of the temperature and of the quantity of the field to be measured, namely within a bandwidth that is not limited by the cycle frequency of the magnetic pulse field, but limited either by the duration of the magnetization-reversing pulse, or by the time constant of the control circuit for compensating the input signal.
According to another implementation of the invention, the cycle frequency at which the reversal or modulation of the magnetization of the magnetoresistive sensor elements takes place, and by which the reversing switches or modulators are controlled, is periodically varied in time. This serves the purpose of preventing conformity from developing over a longer period of time between the frequency of the magnetic field acting on the sensor bridge, and the cycle frequency. If the latter case were to occur, and, furthermore, if the point in time of the change in sign of the magnetic field corresponds with the point in time of the pulse reversing the magnetism, the voltage signals generated by the magnetic field and the voltage signals generated in the same phase by the offset voltage superpose one another. Due to the feedback on the differential amplifier, this would control to zero not only the offset component but also the signal component caused by the input signal, and a measurement would no longer be possible. However, if the cycle frequency of the magnetization reversal is varied within the duration of a period that is shorter than the time constant of the low-pass in the feedback branch of the differential amplifier, such breakdown in the measuring process is excluded at a defined frequency of the measuring signal.
The method as defined by the invention is particularly advantageous when sensor elements are employed that can form an array of sensor elements such as a sensor bridge whose bridge output signal is proportional to the acting magnetic field or to the acting magnetic field gradient if the sensor bridges already contain on the respective chip integrated thin-layer lines for the current for the magnetization and for the compensating current. Such sensors are described, for example in patent specifications U.S. Pat. No. 5,247,278 and U.S. Pat. No. 5,521,501. The thin-layer lines, which are used in said patents instead of coils for generating the respective magnetic fields, have substantially lower inductivities than the compact coils. For example, it is possible in connection with said patents to realize shorter pulses for reversing the magnetism, as well as lower control time constants for compensating the magnetic field to be measured. This makes the bandwidth of the signal processing especially large.